The present invention relates broadly to surgical appliances and, more particularly, to oximetry devices for determining oxygen saturation in a surgery patient's arterial blood.
Over recent years, the measurement of arterial blood oxygen saturation, commonly referred to as oximetry, has come into increasingly widespread usage during surgical procedures as a means for monitoring and preventing undetected hypoxemia of the surgical patient. Essentially, oximetry measures the amount of oxygenated hemoglobin in the arterial blood of the patient as a percentage of the total hemoglobin in the blood.
Various devices, typically referred to as oximeters, are available for performing oximetry measurements. So-called noninvasive pulse oximeters are configured to attach to a patient's fingertip, earlobe or nose and are operable to transmit light of differing wave lengths or colors, typically in the red and infrared spectrums, into the body part and to detect the light transmitted therethrough or the light reflected thereby. It is known that the ability of blood hemoglobin to absorb light varies in relation to the level of oxygenation of the hemoglobin. Accordingly, detection of the reflected or transmitted light from a pulse oximeter indicates the amount of light absorbed, from which the arterial blood oxygen saturation can be calculated.
While non-invasive pulse oximeters of the aforementioned type provide substantial advantages over previous oximetry methods which required the withdrawal of blood samples from a patient, pulse oximeters are still subject to several disadvantages. First, when the patient is in a state of low blood perfusion, e.g. when the patient has lost a substantial amount of blood, is cold, or has peripheral vascular disease or for other reasons does not perfuse the extremities well, difficulty may often be experienced in obtaining a sufficient light transmission or reflectance signal from which to calculate the patient's arterial blood oxygen saturation. Likewise, ambient light sources and relative movement of the patient and the oximeter may also interfere with the accuracy of the measurements and calculations obtained.
During surgery under general anesthesia, it is standard practice to insert an endotracheal tube through the patient's mouth and into the trachea to connect the patient to a ventilator to assist breathing. It is well known that such an endotracheal tube is subject to movement and migration within the patient's trachea which poses a continual potential problem in maintaining correct placement and positioning of the tube within the patient's trachea. Accordingly, it is common practice upon initial insertion of an endotracheal tube for the anesthetist or anesthesiologist to check the patient for equal breathing sounds in both lungs, or to perform a chest radiograph of the patient, or to measure the length of the tube inserted past the patient's teeth or lips in comparison to pre-established norms, as a means of determining whether the tube has been properly positioned. Alternatively, some endotracheal tubes are provided with a metal band positioned to be detectable by a compatible sensor placed on the front of the patient's neck in the suprasternal notch when the tube is properly positioned. Under any of these methods, it is necessary to periodically perform the same check at appropriate intervals over the course of the surgical procedure to insure that the proper positioning of the tube is maintained. Disadvantageously, however, none of these tube placement methods enables continuous monitoring of the placement of the endotracheal tube.